Alignment apparatus for x-ray imaging system

ABSTRACT

A method for aligning a radiation source with a portable image receiver in a radiographic imaging system generates a magnetic field with a predetermined field pattern and with a time-varying vector direction at a predetermined frequency from an emitter apparatus that is coupled to the radiation source, wherein the generated magnetic field further comprises a synchronization signal. Sensed signals from the magnetic field are obtained from a sensing apparatus that is coupled to the image receiver, wherein the sensing apparatus comprises three or more sensor elements, wherein at least two of the sensor elements are arranged at different angles relative to each other and are disposed outside the imaging area of the image receiver. An output signal is indicative of an alignment adjustment according to the amplitude and phase of the obtained sensed signals relative to the synchronization signal.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a Continuation of pending U.S. Ser. No. 13/283,654 filed on Oct.28, 2011, entitled “ALIGNMENT APPARATUS FOR X-RAY IMAGING SYSTEM” whichis a 35 U.S.C. 111(a) application which claims the benefit of twoProvisional applications: (1) U.S. Provisional Application Ser. No.61/450,260, filed 8 Mar. 2011, entitled “ALIGNMENT METHOD FOR X-RAYIMAGING,” by Joseph Stagnitto and (2) U.S. Provisional Application Ser.No. 61/533,396, filed 12 Sep. 2011, entitled “ALIGNMENT APPARATUS FORX-RAY IMAGING SYSTEM” by Joseph Stagnitto et al.

FIELD OF THE INVENTION

This invention generally relates to an apparatus for radiation imagingand more particularly relates to a positioning apparatus for providingproper alignment of the radiation source relative to an image detectiondevice for recording a radiation image.

BACKGROUND OF THE INVENTION

When an x-ray image is obtained, there is generally an optimal alignmentbetween the radiation source and the two dimensional receiver thatrecords the image data. In most cases, it is preferred that the x-raysource provide radiation in a direction that is perpendicular to thesurface of the recording medium. For this reason, large-scaleradiography systems mount the radiation head and the recording mediumholder at a specific angle relative to each other. Orienting the headand the receiver typically requires a mounting arm of substantial size,extending beyond the full distance between these two components. Withsuch large-scale systems, unwanted tilt or skew of the receiver is thusprevented by the hardware of the imaging system itself.

With the advent of mobile or portable radiation imaging apparatus, suchas those used in Intensive Care Unit (ICU) environments, a fixed angularrelationship between the radiation source and two-dimensional radiationreceiver is no longer imposed by the mounting hardware of the systemitself. Instead, an operator is required to aim the radiation sourcetoward the receiver surface, providing as perpendicular an orientationas possible, typically using a visual assessment.

In computed radiography (CR) systems, the two-dimensional image-sensingdevice itself is a portable cassette that stores the readable imagingmedium. In direct digital radiography (DR) systems, the two-dimensionalimage-sensing device is a portable digital detector with either flat,rigid, or flexible substrate support.

FIG. 1 shows a mobile x-ray apparatus that can be employed for computedradiography (CR) and/or digital radiography (DR). A mobile radiographyunit 600 has a frame 620 that includes a display 610 for display ofobtained images and related data and a control panel 612 that allowsfunctions such as storing, transmitting, modifying, and printing of theobtained image. For mobility, unit 600 has one or more wheels 615 andone or more handle grips 625, typically provided at waist-, arm-, orhand-level, that help to guide unit 600 to its intended location. Aself-contained battery pack 626 can provide a power source, eliminatingthe need for operation near a power outlet.

Mounted to frame 620 is a support member 635 that supports an x-raysource 640, also termed an x-ray tube, tube head, or generator mountedon a boom apparatus, more simply termed a boom 70. In the embodimentshown, support member 635 has a vertical column 64 of fixed height. Boom70 extends outward a variable distance from support member 635 and ridesup and down column 64 to the desired height for obtaining the image on aportable receiver 10. Boom 70 may extend outward by a fixed distance ormay be extendible over a variable distance. Height settings for thex-ray source 640 can range from low height for imaging feet and lowerextremities to shoulder height and above for imaging the upper bodyportions of patients in various positions. In other embodiments, thesupport member for the x-ray source is not a fixed column, but is ratheran articulated member that bends at a joint mechanism to allow movementof the x-ray source over a range of vertical and horizontal positions.

In computed radiography (CR) systems, the two-dimensional image-sensingreceiver 10 is a portable cassette that stores the readable imagingmedium. In direct digital radiography (DR) systems, the two-dimensionalimage-sensing receiver 10 is a portable digital detector with eitherflat, rigid, or flexible substrate support. Receiver 10, however,because it is portable, may not be visible to the technician once it ispositioned behind the patient. This complicates the alignment task forportable systems, requiring some method for measuring source-to-imagedistance (SID), tilt angle, and centering, and making it more difficultto use a grid effectively for reducing the effects of scatter. Becauseof this added complexity with a portable radiography system, thetechnician may choose not to use a grid; the result without a grid,however, is typically a lower-quality image.

There have been a number of approaches to the problem of providingmethods and tools to assist operator adjustment of x-raysource-to-receiver angle. One conventional approach has been to providemechanical alignment in a more compact fashion, such as that describedin U.S. Pat. No. 4,752,948 entitled “Mobile Radiography AlignmentDevice” to MacMahon. A platform is provided with a pivotable standardfor maintaining alignment between an imaging cassette and radiationsource. However, complex mechanical solutions of this type tend toreduce the overall flexibility and portability of these x-ray systems.Another type of approach, such as that proposed in U.S. Pat. No.6,422,750 entitled “Digital X-ray Imager Alignment Method” to Kwasnicket al. uses an initial low-exposure pulse for detecting the alignmentgrid; however, this method would not be suitable for portable imagingconditions where the receiver must be aligned after it is fitted behindthe patient.

Other approaches project a light beam from the radiation source to thereceiver in order to achieve alignment between the two. Examples of thisapproach include U.S. Pat. No. 5,388,143 entitled “Alignment Method forRadiography and Radiography Apparatus Incorporating Same” and No.5,241,578 entitled “Optical Grid Alignment System for PortableRadiography and Portable Radiography Apparatus Incorporating Same”, bothto MacMahon. Similarly, U.S. Pat. No. 6,154,522 entitled “Method, Systemand Apparatus for Aiming a Device Emitting Radiant Beam” to Cumingsdescribes the use of a reflected laser beam for alignment of theradiation target. However, the solutions that have been presented usinglight to align the film or CR cassette or DR receiver are constrained bya number of factors. The '143 and '578 MacMahon disclosures require thata fixed Source-to-Image Distance (SID) be determined beforehand, thenapply triangulation with this fixed SID value. Changing the SID requiresa number of adjustments to the triangulation settings. This arrangementis less than desirable for portable imaging systems that allow avariable SID. Devices using lasers, such as that described in the '522Cumings disclosure, in some cases can require much more precision inmaking adjustments than is necessary.

Other examples in which light is projected from the radiation sourceonto the receiver are given in U.S. Pat. No. 4,836,671 entitled“Locating Device” to Bautista and U.S. Pat. No. 4,246,486 entitled“X-ray Photography Device” to Madsen. Both the Bautista '671 and Madsen'486 approaches use multiple light sources that are projected from theradiation source and intersect in various ways on the receiver.

Significantly, the solutions noted above are often of little of no valuewhere the receiver and its accompanying grid are hidden from view, lyingfully behind the patient as may be the case, for example, for chestx-ray imaging with a portable system. Today's portable radiation imagingdevices allow considerable flexibility for placement of the filmcassette, CR cassette, or Digital Radiography DR receiver by theradiology technician. The patient need not be in a horizontal positionfor imaging, but may be at any angle, depending on the type of imagethat is needed and on the ability to move the patient for the x-rayexamination. The technician can manually adjust the position of both theportable cassette or receiver and the radiation source independently foreach imaging session. Thus, it can be appreciated that an alignmentapparatus for obtaining the desired angle between the radiation sourceand the grid and image receiver must be able to adapt to whateverorientation is best suited for obtaining the image. Tilt sensing, as hasbeen conventionally applied and as is used in the device described inU.S. Pat. No. 7,156,553 entitled “Portable Radiation Imaging System anda Radiation Image Detection Device Equipped with an Angular SignalOutput Means” to Tanaka et al. and elsewhere, does not providesufficient information on cassette-to-radiation source orientation,except in the single case where the cassette lies level. More complexposition sensing devices can be used, but can be subject to sampling andaccumulated rounding errors that can grow worse over time, requiringfrequent resynchronization.

Thus, it is apparent that conventional alignment solutions may beworkable for specific types of systems and environments; however,considerable room for improvement remains. Portable radiographyapparatus must be compact and lightweight, which makes the mechanicalalignment approach such as that given in the '948 MacMahon disclosureless than desirable. The constraint to direct line-of-sight alignmentreduces the applicability of many types of reflected light based methodsto a limited range of imaging situations. The complex sensor and motioncontrol interaction required by the Tanaka et al. '553 solution wouldadd considerable expense, complexity, weight, and size to existingdesigns, with limited benefits. Many less expensive portable radiationimaging units do not have the control logic and motion coordinationcomponents that are needed in order to achieve the necessary adjustment.None of these approaches gives the operator the needed information formaking a manual adjustment that is in the right direction for correctingmisalignment, particularly where an anti-scatter grid is used.

A related problem is the need to achieve a source-to-image distance(SID) that is well-suited for the image to be obtained and for the gridused. Conventional alignment solutions do not provide SID information,leaving it to the technician to make separate measurements or to make anapproximate SID adjustment. Moreover, conventional solutions do notprovide the technician with tools to help reduce backscatter, caused bymisalignment or poor adjustment of the collimator blades. This type ofscatter, while not particularly problematic with other types ofradiographic imaging, such as dental and mammographic imaging, can betroublesome with portable radiographic imaging apparatus, since theradiation is directed over a broad area. Radiation that works past theimaging receiver and any blocking element associated with the receivercan inadvertently be reflected back into the receiver, adverselyaffecting image quality. To reduce backscatter as much as possible forchest x-rays and other types of x-ray, the technician is required toestimate the location and orientation or outline of the imaging receiverand to adjust the collimator accordingly.

Significantly, none of these conventional solutions described earlier isparticularly suitable for retrofit to existing portable radiographysystems. That is, implementing any of these earlier solutions would beprohibitive in practice for all but newly manufactured equipment andcould have significant cost impact.

Yet another problem not addressed by many of the above solutions relatesto the actual working practices of radiologists and radiologicaltechnicians. A requirement for perpendicular delivery of radiation,given particular emphasis in the Tanaka et al. '553 application, is notoptimal for all types of imaging. In fact, there are some types ofdiagnostic images for which an oblique (non-perpendicular) incidentradiation angle is most desirable. For example, for the standard chestanterior-posterior (AP) view, the recommended central ray angle isoblique from the perpendicular (normal) by approximately 3-5 degrees.Conventional alignment systems, while they provide for normal incidenceof the central ray, do not adapt to assist the technician for adjustingto an oblique angle.

Thus, it can be seen that there is a need for an apparatus that enablesproper angular alignment and positioning of a radiation source relativeto an image detection device and an optional antiscatter grid forrecording a radiation image.

SUMMARY OF THE INVENTION

An object of the present invention is to advance the art of radiographicimaging by providing exemplary apparatus and methods embodiments to aidin alignment and proper positioning of the radiation source to aradiation receiver. A feature of the present invention is the use of asensor and detector arrangement that is able to sense distance andpositional orientation of the receiver relative to the radiation source.It is an advantage of certain exemplary apparatus and methodsembodiments that can allow straightforward retrofitting for existingx-ray apparatus.

Exemplary apparatus and methods embodiments do not require visibility ofthe receiver behind the patient for alignment. It is a further advantageof one embodiment that it provides a method that can be adapted for usewith a variable SID distance.

These objects are given only by way of illustrative example, and suchobjects may be exemplary of one or more embodiments of the invention.Other desirable objectives and advantages inherently achieved by thedisclosed invention may occur or become apparent to those skilled in theart. The invention is defined by the appended claims.

According to an aspect of the present invention, there is provided amethod for aligning a radiation source with a portable image receiver ina radiographic imaging system, the method can include generating amagnetic field with a predetermined field pattern and with atime-varying vector direction at a predetermined frequency from anemitter apparatus that is coupled to the radiation source, wherein thegenerated magnetic field further comprises a synchronization signal;obtaining sensed signals from the magnetic field from a sensingapparatus that is coupled to the image receiver, wherein the sensingapparatus comprises three or more sensor elements, wherein at least twoof the sensor elements are arranged at different angles relative to eachother and are disposed outside the imaging area of the image receiver;and providing an output signal indicative of an alignment adjustmentaccording to the amplitude and phase of the obtained sensed signalsrelative to the synchronization signal.

BRIEF DESCRIPTION OF THE DRAWINGS

While the specification concludes with claims particularly pointing outand distinctly claiming the subject matter of the present invention, itis believed that embodiments of the invention will be better understoodfrom the following description when taken in conjunction with theaccompanying drawings, wherein:

The foregoing and other objects, features, and advantages of theinvention will be apparent from the following more particulardescription of the embodiments of the invention, as illustrated in theaccompanying drawings. The elements of the drawings are not necessarilyto scale relative to each other.

FIG. 1 shows a perspective view of one type of conventional mobileradiography unit.

FIG. 2A is a perspective view showing the relative relationship of thepatient being imaged to basic components of a diagnostic imagingapparatus.

FIG. 2B is a perspective view showing important dimensionalrelationships for imaging system setup.

FIG. 2C is a perspective view showing exemplary out-of-alignmentpositioning.

FIG. 3A is a perspective view showing the operation of one portion of analignment apparatus in one embodiment.

FIG. 3B is a side view block diagram that shows components used forachieving suitable tube to receiver/grid alignment according to anembodiment of the present invention.

FIG. 4 is a perspective view that shows display of receiver positionaccording to an embodiment of the present invention.

FIG. 5A shows display of a collimator pattern where the radiation sourceis poorly aligned to the receiver.

FIG. 5B shows display of a collimator pattern where the radiation sourceis well aligned to the receiver and anti-scatter grid.

FIG. 5C shows display of a projected collimator pattern where theradiation source is poorly aligned to the receiver.

FIG. 5D shows display of a projected collimator pattern where theradiation source is well aligned to the receiver and grid.

FIGS. 6A and 6B are diagrams that show how projected light patternsalign under various conditions, including centering, angular, anddistance differences.

FIG. 7 is a plan view that shows the use of a display screen coupled tothe collimator for displaying information indicative of the spatialrelation between the radiation source and its receiver.

FIGS. 8A, 8B, and 8C show operator interface examples for use of adisplay screen as a display apparatus.

FIG. 9 shows an operator interface arrangement for the display screen inan alternate embodiment.

FIG. 10 shows a sequence of operator interface display screens for adisplay screen that is mounted near the collimator that changesorientation as the radiation source angle changes.

FIG. 11 is a schematic view showing a radiographic imaging apparatusthat uses an alignment apparatus according to an embodiment of thepresent invention.

FIGS. 12A, 12B, and 12C are schematic diagrams that show basic distanceand angular relationships that relate to signal amplitude and phase at asensor element for an emitted magnetic field.

FIG. 13 is a plan view showing a receiver and anti-scatter grid in aholder that provides sensing elements that form a sensor apparatus.

FIG. 14A is a schematic block diagram that shows components of anemitter apparatus.

FIG. 14B shows waveforms generated by the emitter apparatus of FIG. 14A.

FIG. 14C shows the pattern of the rotating magnetic field vector fromorthogonally disposed sine and cosine coils, as modulated in the emitterapparatus.

FIG. 15A is a schematic block diagram that shows components of a sensorapparatus.

FIG. 15B shows waveforms acquired by the sensor apparatus of FIG. 15A.

FIG. 15C shows the demodulated and filtered waveforms of FIG. 15B,processed by the sensor apparatus.

FIG. 16A is a logic flow diagram that lists basic steps for alignmentaccording to an embodiment of the present invention.

FIG. 16B is a logic flow diagram that shows the operation sequence foralignment using the apparatus of the present invention.

FIG. 17 is a logic flow diagram that shows an alternate sequence forautomated alignment using the apparatus of the present invention.

FIG. 18 is a logic flow diagram that shows how control logic executes afield generation and processing step according to an embodiment of thepresent invention.

FIG. 19 is a logic flow diagram that shows processing for terminationstep according to an embodiment of the present invention.

FIG. 20 is a logic flow diagram that shows periodic processing of sensordata according to an embodiment of the present invention.

FIG. 21 is a side view schematic that shows measurement considerationsrelated to a metal patient bed.

FIG. 22 is a graph that relates relative position offset to grid frameangle.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

Unlike the limited tilt sensing approaches that have been used in avariety of earlier radiography systems, certain exemplary apparatus andmethods embodiments provide a straightforward solution to the problem ofradiation source-to-receiver alignment that can be used with a number ofx-ray imaging systems.

In the context of the present disclosure, the term “imaging receiver”,or more simply “receiver”, may include a cassette that has aphotostimulable medium, such as a film or phosphor medium, for example,or may include a detector array that records an image according toradiation emitted from the radiation source. A portable receiver is notmechanically coupled to the radiation source, so that it can be easilyand conveniently positioned behind the patient.

As used herein, the term “energizable” indicates a device or set ofcomponents that perform an indicated function upon receiving power and,optionally, upon receiving an enabling signal.

In the context of the present disclosure, two elements are considered tobe substantially orthogonal if their angular orientations differ fromeach other by 90 degrees+/−12 degrees.

In the context of the present disclosure, the term “coupled” is intendedto indicate a mechanical association, connection, relation, or linking,between two or more components, such that the disposition of onecomponent affects the spatial disposition of a component to which it iscoupled. For mechanical coupling, two components need not be in directcontact, but can be linked through one or more intermediary components.

The perspective view of FIG. 2A shows components of a radiographicimaging apparatus 30. A radiation source 20, such as an x-ray source,directs radiation toward a patient 14. A receiver 10 positioned behindthe patient forms the diagnostic image from the incident radiationpassing through patient 14. Receiver 10 may have a photostimulablemedium, such as a film or phosphor medium, for example, or may have adetector array that records an image according to radiation emitted fromradiation source 20. Receiver 10 may have landscape or portraitorientation. An optional antiscatter grid 12 has plates 18 arranged asshown in FIG. 1A, just above the surface of the receiver 10. Radiationsource 20 has a collimator 22 that defines the radiation field that isdirected outward from source 20, toward receiver 10 in the example ofFIG. 2A.

Radiation source 20 has an adjustable angular orientation for directingradiation toward receiver 10. FIG. 2B (with patient 14 not shown forbetter visibility of system components) shows coordinate xyz axes. Here,the source-to-image distance (SID) is in the general direction of the zaxis. In FIG. 2B, radiation source 20 is in its aligned position, at asuitable SID from receiver 10. Grid plates 18 are angularly arranged sothat they define a focal line L where their respective planes convergeat the SID. For best alignment for most imaging in such an embodiment,radiation source 20 should be centered near focal line L and have theface portion of collimator 22 generally parallel to the planar surfaceof receiver 10. However, there can be image types for which a slightangular offset is preferred.

FIG. 2C, by contrast, shows phantom outlines at 20′ and 20″ for poorpositioning of radiation source 20. At positions 20′ and 20″ shown inphantom, the SID is almost acceptable; however, radiation source 20 isnot centered near focal line L and its angular orientation is badlyskewed. Alignment of the radiation source with the antiscatter gridwould be poor at these and similar out-of-alignment positions, degradingimage quality or, at worst, preventing a suitable diagnostic image frombeing obtained.

The perspective view of FIG. 3A and side view of FIG. 3B show the use ofan alignment sensing apparatus 40 that is energizable to sense therelative spatial relationship between radiation source 20 having aradiation path represented as path P and distributed about a centralaxis and imaging receiver 10 sensitive to radiant energy and positionedadjacent the subject for forming the radiographic image and to generateone or more output signals indicative of the relative spatialrelationship, including alignment and distance. In the embodiment shownin FIGS. 3A and 3B, a holder 46 has one or more sensor elements 42(e.g., electromagnetic coils) that are disposed outside the imaging areaof the image receiver and that receive an electromagnetic field havingtime-varying vector directions or signal that is generated by a singleemitter T or a pair of emitters T1 and T2, each emitter T, T1, or T2comprising field generation elements 44, shown coupled to the radiationsource 20, such as mounted near collimator 22 or generated by a motorthat rotates a magnet or energized coil. In one embodiment, the sensorelements 42 are not co-planar. In one embodiment, the field generationelements 44 are not co-planar.

Holder 46 also holds receiver 10. In an alternate embodiment, alignmentsensing apparatus 40 components are built into receiver 10. In yetanother alternate embodiment, the signal direction is reversed: signalsare generated from one or more emitters T, coupled to receiver 10 onholder 46 and detected by sensor elements coupled with source 20. Anadditional sensor 28, such as an inclinometer, accelerometer, compass,gyroscopic, or other device for obtaining an angular measurement can beprovided on either or both receiver 10 or radiation source 20. The oneor more position-sensing signals from alignment sensing apparatus 40 goto a control logic processor 48 that provides the control logic fordetermining whether or not adjustment is needed and, optionally, forproviding this information to a display apparatus 52, such as a displayscreen mounted on radiation source 20 as shown in FIG. 3B. In analternate embodiment, an optional projector 54 is provided forprojecting a display onto the patient or other subject indicating thestatus of the alignment and providing guidance on the particularadjustment that is needed.

Optional projector 54, shown mounted on the x-ray source 20 in FIG. 3Band following, may be a pico-projector, such as a Pico Projector Displayfrom Microvision Inc., Redmond, Wash., USA, or a Micro Projector fromAAXA Technologies, Inc., Santa Ana, Calif., for example. Image formingdevices such as these are advantaged for a number of reasons, includingsmall size, low weight, and low power requirements. Thesesmall-footprint projectors, currently used in cell-phone and otherhighly portable electronic devices, scan one or more low-powersolid-state light sources, such as light-emitting diodes (LEDs) orlasers onto a display surface. This type of projector requires a smallnumber of optical components for projection over a range of distances.The solid-state light source itself can typically be turned on and offrapidly as needed, so that power is consumed only for those image pixelsthat are projected. This allows the display device to operate at lowpower levels, so that battery power could be used for projector 54.Alternate embodiments use other types of electronic imaging projectorsas image forming apparatus, such as those that employ a digitalmicromirror array such as the Digital Light Processor (DLP) from TexasInstruments, Inc.; an array of micro-electromechanical grating lightvalves, such as the Grating Light Valve (GLV) device from Silicon LightMachines, Inc.; or a liquid crystal device (LCD) including a LiquidCrystal on Silicon (LCOS) device. In an alternate embodiment, projector54 is provided by a light source and a movable target, with a motor orother actuator that moves the target, where the target is positioned inthe path of the light source for providing an image that shows thereceiver location.

The perspective views of FIG. 4 show how optional projector 54 performsthe display function according to one embodiment of the presentinvention. Projector 54 can project light to form images over an imagefield 58 that exceeds the area of receiver 10, as shown at left. Whenreceiver 10 is located using alignment sensing apparatus 40, projector54 displays a receiver pattern 60 on patient 14, wherein receiverpattern 60 indicates at least an outline showing the location ofreceiver 10 behind or underneath patient 14. At the right, the desiredalignment is shown, wherein a collimator pattern 62, emitted from thecollimator light source in the x-ray tube head, is aligned with receiverpattern 60. Notably, with this arrangement, projector 54 can project animage over an area that exceeds the size of receiver 10, enabling theoutline of receiver 10 to be displayed prior to centering of thecollimator and radiation path onto receiver 10.

The perspective view of FIG. 5A shows collimator pattern 62 that isdisplayed from radiation source 20 in a spatial arrangement wherein theradiation path of radiation source 20 (centered along axis P asdescribed previously) is not aligned with receiver 10 or its grid 12.The perspective view of FIG. 5B shows projector 54 in display apparatus50, projecting receiver pattern 60 directly at receiver 10. FIG. 5Cshows the overlaid paths and mismatched patterns 60 and 62 that indicatepoor alignment between radiation source 20 and receiver 10. Theperspective view of FIG. 5D then shows correct alignment, whereinreceiver pattern 60 and collimator pattern 62 are center-aligned andsymmetrical. It can be observed that parallax problems between projector54 and the collimator pattern 62 can be encountered when the SID isincorrect, with receiver 10 either too far or too near with respect toradiation source 20.

The positional relationship of displayed patterns from projector 54 andfrom the collimator light of the x-ray tube head can be used asindicators of alignment. By way of example, FIG. 6A shows how alignmentof collimator pattern 62 from the collimator light with receiver pattern60 from projector 54 indicates needed alignment adjustment of radiationsource 20 with its receiver 10. The patterns shown at 60 and 62 arerepresentative examples selected for illustration and can take any of anumber of forms, including, but not limited to, crosshair patterns,including crosshair patterns with or without a central circle as shownin the example of FIGS. 6A and 6B. At a relative position 90, source 20and receiver 10 are not aligned and respective patterns 62 and 60indicate this misalignment. At a relative position 92, source 20 iscloser to alignment with receiver 10, closer to centering than shown atposition 90, and patterns 62 and 60 display as somewhat overlapping butare not centered with respect to each other. At a relative position 94,source 20 and receiver 10 are aligned and the displayed respectivepatterns 62 and 60 are overlaid to indicate this centering alignment. Inaddition, position 94, with both patterns 60 and 62 at the same size andover substantially the same area, also indicates that the collimator hasbeen properly set to limit the radiation distribution and to reduce thelikelihood of backscatter. Values 66 for SID and angle are alsodisplayed by projector 54. In an alternate embodiment, asource-to-object distance (SOD) also displays. The projected values canbe positioned within or outside receiver pattern 60. In alternateembodiments in which collimator blade position can be sensed, additionalinformation on properly sizing and orienting the collimated light beamcan also be provided in the display.

FIG. 6B shows other examples that represent poor relative positioning ofsource 20 and receiver 10. In a relative position 96, source 20 isnearly centered with respect to receiver 10, but the angle is skewedfrom normal. Receiver pattern 60 is accordingly non-rectangular, such ashaving a keystone pattern, for example, indicating the angularrelationship of the radiation path from source 20 and receiver 10. In arelative position 98, source 20 is nearly centered with respect toreceiver 10, but either the source-to-image distance (SID) is incorrector, if correct, the collimator should be adjusted to reduce backscatter.In this case, the respective patterns 60 and 62 appear to be ofdifferent sizes to indicate the need for SID adjustment.

Where projection is used for display apparatus 50, in addition to thereceiver 10 outline, information of various types can be displayed on oralongside the patient, for example, location of the receiver, locationof automatic exposure control (AEC) device, grid information, actual andrecommended SID, patient information, and some portion of the alignmentinformation.

Display Screen 52 as Display Apparatus 50

FIG. 7 shows display screen 52 that can supplement or substitute forprojector 54 in an alternate embodiment of display apparatus 50. In oneembodiment, display screen 52 is mounted near collimator 22 as shown, sothat the operator can view displayed results while moving radiationsource 20 into position. In alternate embodiments, the alignment utilitymay be provided on a removable or remote display screen or on display610 (FIG. 1), the display console that is part of radiographic imagingapparatus 30 itself.

FIGS. 8A, 8B, and 8C show operator interface examples when using displayscreen 52 as display apparatus 50. Various graphical icons and imagesare used to symbolize the adjustments needed for proper centering,angulation, and SID. An angle adjust indicator 100 provides variousgraphical and measured data to help guide proper angular adjustment ofthe source 20 to receiver 10. Angular information displays one or moreof the following:

-   -   (i) Receiver angle. An angular measurement relative to true        horizontal can be obtained from the optional inclinometer or        other sensor 28 (FIG. 3B) or from other alignment sensing        apparatus 40 data.    -   (ii) Tube angle for radiation source 20. This angular        measurement relative to true horizontal can similarly be        calculated from inclinometer or other sensor 28 or other        alignment sensing apparatus 40 data.    -   (iii) Receiver/grid to source 20 angle. This relative angular        measurement between receiver 10 and source 20 can be obtained        using measurements from one or more optional sensors 28 (FIG.        3B) or from other alignment sensing apparatus 40 data.    -   (iv) Intercept angle data for source-to-grid 12 alignment.    -   (v) Source to receiver angle relative to desired angle,        calculated from alignment sensing apparatus 40 measurements.        This includes adjustment for non-normal angles.

A SID indicator 110 lists not only the current SID value obtained frommeasured data, but, in the embodiment shown, also shows the amount ofadjustment needed. A centering indicator 120 provides text and graphicalinformation on centering error and needed adjustment direction. In FIG.8B, centering indicator 120 includes a graphic element 104 that showsthe portrait/landscape orientation of the receiver. Icons 102 use color,animation, including flashing or video clips, and symbols of differenttypes to indicate the needed adjustment direction for the correspondingvalue. Graphic elements 104 are also provided to help visually indicatethe adjustment needed. Graphic elements 104 can be any of a number oftypes of suitable element, including circles, bars, or other shapes.Color can be used to indicate correct angular, centering, or distancevalues, with differences in color indicating the recommended directionof needed change, if any, and color transitions indicating movementbetween positions. Various thresholds are used to determine how close anadjustment is to a desired setting.

FIG. 9 shows a plan view of an alternate embodiment for the operatorinterface on display screen 52. SID indicator 110 lists the current SIDvalue obtained from measured data. Here, graphic elements 104 includesliders that show the relative amount of adjustment that is needed forcentering, distance, and angle. Centering of the slider indicatescorrect positioning. Angle adjust indicator 100 shows the measuredangular values for the receiver or x-ray source relative to truehorizontal or, optionally, relative to each other or to a preferredsetting. In an optional embodiment, the difference between theirrelative angles is displayed. Centering indicator 120 shows an image oroutline of receiver 10, such as at portrait or landscape orientation,with a superimposed icon 122 that shows the relative position and shapeof the x-ray beam. Control buttons 124 provide useful utilities forimproving alignment, obtaining information about the system or aboutsystem components, and other functions. In an alternate embodiment, oneof the control buttons 124 is used to set up the view type for theupcoming radiographic image (such as, for example, an AP chest exam viewtype) and to indicate the type of grid used, if any. This setup can thencause specific SID and angle values to be assigned and displayed for theimage.

FIG. 10 shows a sequence of operator interface display screens for adisplay screen 52 that is mounted near collimator 22 and that changesorientation as radiation source 20 angle changes. At a position 130, areceiver icon 132 displays, along with a centering target icon 134 and aradiation source icon 136. At a position 140, centering is partiallyachieved, but the radiation source 20 must be redirected toward thereceiver. At a position 150, radiation source 20 is being turned and thescreen display dynamically re-orients itself to represent positions ofcomponents with receiver icon 132 and icons 134 and 136. A SID icon 152graphically shows that radiation source distance to the receiver must beadjusted. SID icon 152 changes position as the SID changes. At aposition 160, proper centering, angle, and SID are obtained. The SIDvalue displays as shown at SID indicator 110.

In one embodiment, sensors are also able to indicate whether or not grid12 is used and, if so, the type of grid 12 that is being used. Thesystem can then display information such as Transverse or Longitudinalgrid type; Grid ratio, for example: 6:1, 8:1, 10:1; optimal SID (or SIDrange) for the grid being used; and indication or message to use thecorrect grid type (transversal or longitudinal) based on detectedrotation of the receiver. If the patient is not lying flat, the systemcan determine this through the grid's inclinometer data, and can alsodetermine this condition using other sensor data. The system can alsoprovide a warning message related to grid cutoff, a condition thatoccurs when the angle of the radiation path is excessively skewed to oneside or the other of the grid, causing the grid elements to block asubstantial amount of radiation. When the presence or absence of a gridis determined, system logic can automatically select the correct viewfor the exam or change the existing view to a different one. Forexample, the system can switch from a non-grid view to a grid view. Thisnew view may have a different name, different exposure parameters ortechniques, and different image processing parameters. In an alternateembodiment of the present invention, the image type or view isdetermined and one or more appropriate settings for centering, angle,and SID are automatically assigned based on the view type. The view canbe set up by the operator, such as using display screen 52 and mayspecify the type of grid used. Alternately, the view can be determinedfrom measured data, such as inclinometer readings, for example.

Alignment Apparatus

As noted previously, alignment is of particular value when a grid 12(FIG. 2A, for example) is used with the imaging receiver. Alignment ofthe source to the grid helps to improve the obtained image by reducingscatter.

The schematic view of FIG. 11 shows radiographic imaging apparatus 30using alignment apparatus 40 and labels the two translation and threerotational stages plus collimator rotation needed for alignment. In theembodiment of FIG. 11, a single emitter apparatus T is used foralignment. Embodiments with a pair of emitter apparatus T1 and T2 arealso used.

Embodiments of the present invention provide positional information forsource-to-detector alignment using emitter apparatus T that generates amagnetic field with a predetermined position field pattern relative tothe mounting arrangement used for the emitter components and withcontinuously varying or time-varying vector direction and comparingsignals sensed from the varying magnetic field by a set of sensors thatare arranged at different positions. The predetermined position fieldpattern that is emitted can be the same pattern under any imagingconditions and does not vary in position relative to the position of theimage receiver, but has the same position relative to the emittercircuitry and is dependent on the spatial position in which emitter Tapparatus components 44 are mounted. For example, where a motor is usedto generate the field pattern, the same rotated field pattern is usedeach time, as determined by the position of the rotated field generationelement 44 and the motor axis. Where separate sine and cosine coilsgenerate the field pattern as part of emitter apparatus T, this patternis in fixed position relative to these components and this position doesnot change. These same components can also be used to provide thesynchronization signal needed for using the detected sensor signals todetermine relative position.

The schematic block diagram of FIG. 12A shows how this alignmentmechanism sensing can operate. At an emitter apparatus T, a signalgenerator 180 generates, about itself, a time-varying magnetic fieldthat has a periodic varying magnetic vector pattern, with a fixed fieldposition pattern 56, at a predetermined frequency. At a sensingapparatus R, sensor elements 42 a, 42 b, and 42 c are spread apart andat different angles with respect to the generated field. Each of sensorelements 42 a, 42 b, and 42 c obtains a corresponding signal 84 a, 84 b,and 84 c respectively. Signal amplitude at each sensor at any instancein time is indicative of the distance between emitter T and the sensorelement and is also a factor of the direction of the sensor's highestsensitivity relative to the direction of the magnetic field vector.Where sensor elements 42 a, 42 b, and 42 c are coils, for example, thedirection of the sensor's highest sensitivity relates to the axis ofsymmetry A of the core of the coil.

As shown in FIG. 12A, signals 84 a, 84 b, and 84 c from sensor elements42 a, 42 b, and 42 c are shifted in phase φ1, φ2, and φ3 from each otherdue to their relative angles with respect to the time varying magneticfield vector. A synchronization or sync signal 86 is generated from theemitter and is used to relate timing of the sensor signals to theorientation of the magnetic field vector in the emitter's frame ofreference. This combination of sync signal timing with phase andamplitude information from each sensor element provides informationuseful for obtaining the relative positions of x-ray source anddetector.

Emitter T can generate the magnetic field with time-varying vectordirection in a number of ways. Where rotary motion is provided, a singlecoil or other field generating element can be used. FIG. 12B shows frontand side views of an embodiment of signal generator 180 in emitter Twith a motor 88 that spins a magnet 72 in order to generate thetime-varying magnetic vector pattern that is needed. In one embodiment,magnet 72 is a permanent magnet. In an alternate embodiment, magnet 72is a coil having a direct current (DC) drive.

Magnet 72 can also be a coil that is driven by an alternating current(AC) signal. In such an embodiment, the AC signal acts as a type ofcarrier signal, with the time varying vector direction change providedby the rotation from motor 88. Each sensor sees an amplitude modulationof the carrier signal over time, according to the sensor's direction ofhighest sensitivity relative to the rotational angle of the motor.Advantageously, using the AC signal allows tuning of the sensor elementsin sensing apparatus R to have a higher gain at the frequency of the ACsignal. This can provide an improved signal to noise ratio. In general,a carrier signal is at a frequency higher than the frequency of themagnetic field in space with time-varying vector directions that isgenerated by the emitter apparatus.

The time-varying magnetic field can also be formed using signalgenerator 180 that comprises two or more stationary coils or otheremitters that do not require rotation but that cooperate to emit amagnetic field vector that is time-varying and can be detected by sensorelements. The time-varying magnetic field is generated by modulating themagnitudes of the field from each coil, with respect to time, in asynchronized manner. FIG. 12C shows front and side views of an alternateembodiment for signal generator 180 in emitter T, using two coils 182 aand 182 b disposed orthogonally or with some other angular differencebetween them. Coils 182 a and 182 b can be in different planes, as shownin FIG. 12C.

There are six degrees of freedom (DOF) for consideration insource-detector positioning. To determine relative positioning, at leastsix independent position-related measurements are required. A solutioncan be found using at least one emitter T generating a time varyingmagnetic field vector, with three or more sensor elements at the sensingapparatus R. This arrangement can provide both a phase and magnitudemeasurement on each of the three sensing elements to provide therequired six independent measurements to solve for six unknown degreesof freedom. Alternately, two emitters T1 and T2, each generating timevarying magnetic field vectors, could be used, with two or more sensorelements at sensing apparatus R in order to get sufficient independentmeasurements to determine the source-detector positioning.

Sync signal 86 is provided by emitter T or, alternately, from someexternal timing mechanism. This signal can be provided in the magneticfield itself, such as by generating a short pulse magnetic signal orother timed signal or may be provided using a wired signal connection,such as a signal from control logic processor 48, for example.

The plan view of FIG. 13 shows receiver 10 and its grid 12 seated withinholder 46. Using the principle described in FIGS. 12A, 12B, and 12C foremitted and sensed magnetic fields, the direction of the generatedmagnetic field is sensed using the apparatus of FIG. 13 and used toindicate the relative alignment of each sensor element 42 to emitterapparatus 44. The receiver signals are synchronously measured from eachof the sensor elements 42 and recorded. Sensor elements in the sensingapparatus are spatially separated to take advantage of signaltriangulation. Sensor elements 42 can be peripheral to image receiver 10and lie outside the imaging area of the image receiver 10, as shown inFIG. 13. Where sensor elements 42 are coils, their axes of symmetry,(the same as the axis of symmetry of each core element of the coil), liein planes that are generally parallel to the planar surface of receiver10. Sensed magnitude and phase information is evaluated mathematicallyto derive the relative position and orientation between emitters andsensor elements. Associated as part of each sensor element 42, describedsubsequently in the present disclosure, is supporting signalamplification and measurement circuitry, along with components formaintaining signal communication with control logic processor 48 (FIG.3B).

The use of stationary emitter coils, as described with reference to FIG.12C, is advantaged by eliminating the need for motor or other actuatorwith moving parts to provide the time-varying magnetic field by rotatingthe coil or other field generation element 44. As was shown in FIGS. 3Aand 3B, for a stationary emitter T, there can be at least two fieldgeneration elements 44 on radiation source 20, with the two fieldgeneration elements 44 that form each emitter T typically disposedorthogonally with respect to each other. Field generation elements canalternately be at any known relative orientation and spacing apart fromeach other. Field generation elements 44 are typically paired coils.

It should be noted that additional field generation elements 44 andadditional sensor elements 42 can be used to advantage for positiondetection, using strategies such as disposing sensor elements 42 atother angles. According to one embodiment herein, three or more sensorelements 42 are provided, with adjacent elements rotated at 45 degreeincrements with respect to each other. This allows additional signalinformation to be available, so that more accurate positional andorientation measurement can be obtained. Further, the relative positionsof sensor and field generation elements 42 and 44 could be reversed fromthat shown, so that field generation elements 44 of emitter apparatus Tare on holder 46 and sensor elements 42 disposed on radiation source 20.To help reduce induced surface current effects, coils for generation andsensing are aligned substantially parallel to nearby metal surfacestructures, such as parallel to the metallic case of the DR receiver 10or parallel to collimator features, for example. For a sensor coil,substantially parallel alignment means alignment of the core axis towithin about 10 degrees of parallel, preferably within no more thanabout 2 degrees from parallel. For other types of sensor devices,substantially parallel alignment means alignment of the highest axis ofsensitivity to within about 10 degrees of parallel or less.

The schematic block diagram of FIG. 14A shows an emitter circuit 200 forgenerating and emitting the time varying magnetic field from a pair ofemitter apparatus T1 and T2 according to an embodiment of the presentinvention. Each emitter apparatus T1 and T2, in the embodiment shown inFIG. 14A can modulate separate field generation elements, which caninclude a sine coil 43 and a cosine coil 45. A control processor 202, insignal communication over a communications link 204 with other controllogic, such as control logic processor 48 (FIG. 3B), coordinates controlof this modulation with a digital-to-analog (D/A) converter 208 andoscillator 210 to provide signals to appropriate voltage-to-currentamplifiers 212 that drive the coils of field generation elements 44.Control processor 202 provides a control circuit that can alternatelyenergize each emitter apparatus T1, T2 during a discrete time interval,so that both pairs of field generation elements, sine and cosine coils43 and 45 respectively, are not emitting a magnetic field at the sametime. An optional oscillator 210 provides a carrier frequency that isamplitude-modulated within a waveform envelope, such as a sinusoidalenvelope, according to one embodiment of the present invention. Therespective coils in sensor and emitter apparatus T elements are tuned tothe carrier frequency. Selection of a suitable carrier frequency can bebased on a number of factors, including power and distanceconsiderations, relative degree of penetration through the patient,interference with nearby equipment, signal-to-noise ratio, and otherfactors. Consistent with an embodiment of the present invention, acarrier frequency ranging from several kHz to several MHz can be used.In general, the carrier signal is at a frequency higher than thefrequency of the magnetic field with time-varying vector directionsgenerated from the emitter apparatus.

To take advantage of the relationship of phase to position describedwith reference to FIGS. 12A-12C, the emitted magnetic field is amodulated signal that varies over time, such as a sinusoidal or otherrepeatable or periodic signal. This field is typically emitted from oneemitter apparatus T1, T2 at a time and repeats as often as is necessaryto help establish the spatial relationship between the x-ray source andreceiver. In the embodiment of FIG. 14A, for example, each emitterapparatus T1, T2 can emit its magnetic field over a separate timeinterval. Emission by the first emitter apparatus T1 terminates, or isin the process of being terminated, before emission by the secondemitter apparatus T2 begins, so that their respective time intervals formagnetic field generation are at least substantially non-overlapping.Sine coil 43 and cosine coil 45 can be spatially separated by a variabledistance, such as in the same plane, or may be disposed in differentplanes. Spatial separation of sine coil 43 and cosine coil 45 can helpto improve signal processing accuracy, as is useful for triangulation,for example.

Use of a carrier signal is optional, but has advantages for detection ofthe time-varying magnetic field over a distance and in a noisyenvironment. Sensor elements 42 can be tuned to the carrier frequency,to improve the signal-to-noise ratio. The wave forms shown in FIG. 14Bshow the modulated carrier frequency in a sinusoidal envelope, asemitted signal 82, detected at sensor elements 42, formed by summingsignals from paired sine and cosine coils. By way of exemplary schematicillustration and example, FIG. 14C shows the pattern of the rotatingmagnetic field vector from orthogonally disposed sine and cosine coils,as modulated in the emitter apparatus.

The schematic block diagram of FIG. 15A shows a sensing apparatus 220for obtaining and conditioning the emitted signal from emitter circuit200. Each sensor element 42 can be a coil according to one embodiment ofthe present invention. Other embodiments employ some other device assensor element 42, such as a Hall-effect sensing device, amagneto-resistive sensor, a Giant magneto-resistive (GMR) sensor, orFlux Gate sensor, for example. In an embodiment that uses a coil, thecoil of each sensor element 42 can provide its signal to a pre-amplifier222 and a band-pass filter 224 for noise removal. The signal can then beprocessed by an amplifier 226 and a demodulator 228. The demodulatedsignal can be provided through a low-pass filter 230 to ananalog-to-digital (A/D) converter 232. A control processor or othersignal generation circuit 236, in signal communication over acommunications link 238 with other control logic such as control logicprocessor 48 (FIG. 3B), is then energizable to generate an outputsignal, such as an output data signal that is indicative of the positionand orientation of the sensing apparatus relative to the emitterapparatus, as well as the distance between the sensing and emitterapparatus. The function of signal generation circuit 236 can beperformed by a dedicated microprocessor, as shown in FIG. 15A, or bysome other circuit, such as an analog circuit, for example. Othercomponents for indicating device orientation, such as an accelerometer,for example, can also be in signal communication with signal generationcircuit 236.

As noted previously, emitter circuit 200 can be coupled to the x-raysource 20, such as installed at collimator 22, with correspondingsensing apparatus 220 as part of holder 46. In an alternate embodiment,the reverse arrangement can be used, with emitter circuit 200 coupled toreceiver 10 and installed at holder 46 and sensing apparatus 220 coupledto x-ray source 20.

FIG. 15B shows exemplary received waveforms from sensing apparatus 220for each coil of a set of sensor elements 42. The left half of thisfigure shows waveforms relating to the signal from emitter T1; the righthalf shows waveforms from emitter T2. Each row is the signal for onesensor element 42 coil. As can be seen from this figure, each sensorsignal can have a different magnitude and phase for the same generatedtime-varying magnetic field, wherein the magnitude and phase depend onrelative sensor position and orientation. Synch signal 86 can also bedetected, as shown. FIG. 15C shows exemplary demodulated waveforms afterprocessing through the circuitry of FIG. 15A.

Operation Sequence for Alignment

The logic flow diagram of FIG. 16A lists basic steps for alignmentaccording to certain exemplary embodiments. In a signal generation step250, one or more magnetic fields with predetermined field pattern andtime-varying vector direction can be generated from emitter apparatus Tor, alternately, from emitter apparatus T1 and T2. A sync signal canalso be generated as part of field generation. A signal sensing step 260obtains sensed output signals from the generated magnetic field from asensing apparatus that is coupled to the image receiver. The sensingapparatus has three or more sensor elements, wherein at least two of thesensor elements are arranged at different angles relative to each otherand can be disposed outside the imaging area of the image receiver, asdescribed previously. In an output signal step 270, an output signal isprovided. This signal is indicative of an alignment adjustment,according to the amplitude and phase of the obtained sensed outputsignals relative to the synchronization signal. Subsequent exemplarydescriptions provide additional detail related to this process.

The flow diagram of FIG. 16B shows an operation sequence for alignmentthat can use or be implemented in exemplary apparatus embodimentsherein. In a positioning step 300, the operator positions radiationsource 20 pointing generally toward receiver 10, which is positionedbehind the patient. The operator then enters a command that initiatesrepeating cycles of field generation and processing. Field generationand processing step 310 provides automatic generation of the sync signal86 and magnetic field from each respective emitter apparatus T1 and T2in sequence and sensing and processing of each received signal fromsensor element 42, using the circuitry and signal sequence describedwith reference to FIGS. 14A through 15C. In an assessment step 320, theoperator views displayed guidance data for readjustment of x-ray source20 position. If adjustment is needed, an adjustment step 350 is executedand assessment step 320 repeated as shown. If a decision step 330indicates success, alignment signaling and processing ends in atermination step 340. In the processing sequence, each respectiveemitter apparatus T1, T2 can be individually energized, in sequence,over substantially non-overlapping time intervals, with the cycleinitiated and repeated as many times as controlled by control circuitsuch as control processor 202 (FIG. 14A). With each generation of atime-varying magnetic field by either emitter apparatus T1 or T2, someor all of the sensor elements 42 can be sensed for received signal.

Automated processing uses control logic to provide motor control signalsfor adjusting the position of radiation source 20. The flow diagram ofFIG. 17 shows an alternate sequence for automated alignment that can usean apparatus embodiment. In a positioning step 300, the operatorpositions radiation source 20 pointing generally toward receiver 10position behind the patient. The operator then enters a command thatinitiates a field generation and processing step 310 that providesautomatic generation of the magnetic field from each respective emitterapparatus T1, T2 in sequence and sensing and processing of each receivedsignal from sensor element 42, using the circuitry and signal sequencedescribed with reference to FIGS. 14A through 15C. In an automatedassessment step 322, control logic in control logic processor 48 orlocal to holder 46 or other component then determines what adjustmentsare needed. If a decision step 332 indicates success, a reporting step338 indicates this to the operator. Alignment signaling and processingends in a termination step 340. If adjustment is needed, an automatedadjustment step 352 is executed by control logic and assessment step 322is repeated as shown.

The flow diagram of FIG. 18 shows how control logic executes fieldgeneration and processing step 310 according to another exemplaryembodiment. In a signal initiation step 312, emitter circuit 200 isinstructed to begin generation of the emitted signal from each fieldgeneration element 42 in sequence. In a sensing request step 314, thesensing apparatus 220 components are polled to obtain the sensedsignals. A processing step 316 then executes to continuously obtain andprocess the sensed signal output from sensing apparatus 220.

The flow diagram of FIG. 19 shows processing for termination step 340according to an exemplary embodiment. Three request steps 342, 344, and346 are sent, to respectively de-activate field generation, magneticfield vector sensing, and periodic processing of received sensor elementdata, respectively.

The flow diagram of FIG. 20 shows processing step 316 for periodicprocessing of sensor data from sensing apparatus 220 according to anexemplary embodiment. In a request step 362, the computer or controllogic processor 48 prompts sensing apparatus 220 for sensor data. Anevaluation step 364 checks the signal strength. In a decision step 366,signal strength is compared against a desired threshold or other measureto determine whether or not to adjust either emitter circuit 200 outputpower or gain circuitry on sensing apparatus 220 in an adjustmentrequest step 370. When signal strength is acceptable, control logicacquires the time-varying signals from each sensor element 42 in sensingapparatus 220.

In an alignment matching step 372, control logic compares the acquiredsignals against a mathematical model of theoretical phase and amplitudefor the relative spatial disposition, including orientation anddistance, of receiver 10 relative to source 20 until a best-fit match isidentified. This match then provides the data needed to closelyapproximate the relative positions. A reporting step 374 then providesadjustment guidance to the operator, such as using the display screensdescribed previously. It should be noted that processing step 316continues during operator adjustment of x-ray source 20 position,updating the position report that is provided as the operator correctsthe source positioning.

In practice, when the sensor elements 42 are attached to the grid frameand the whole assembly that includes receiver 10 is placed on a patientbed for imaging, the metallic structure of the bed itself, or of othertypes of surrounding metal structure associated with the patient's bed,can present interference to the alignment system. The level ofinterference varies with bed type, position and orientation of the frameon the bed, angle of the movable sections of the bed, and relativeproximity of the alignment emitter.

This interference related to the bed structure can be caused by magneticfields from eddy currents induced by the primary emitter fields inelectrically conductive surfaces. A metal sheet surface can act as amagnetic “mirror” for the primary emitter fields. In addition, eddycurrents tend to propagate toward perimeter edges of the sheet. This cancause intensified fields near the edges that potentially produce moreinterference. The interference can also be caused by magnetic fieldsfrom currents induced in electrically conductive loops, such as sectionsof the metal support structure and metal bed rails. Further, theinterference can be caused by magnetic field distortion from ferrousmetals that are disposed anywhere near the path of magnetic fields thatare emitted by the alignment system emitter apparatus T and measured bythe sensing apparatus R coils. The electric currents that are induced inthe conductive metal surfaces and loops by magnetic fields from theemitters have surrounding fields that can also be detected by the sensorelements. Nearby ferrous metals can alter the normal expected path formagnetic field lines between the emitter apparatus and the sensingapparatus. These types of interference can cause phase shifts andamplitude modifications to the signals being observed by the grid framesensors. This can result in measurement error of the alignment system.

The interference caused in this manner can be mitigated using any of anumber of methods. One method is to apply compensation based onempirical data. For example, FIG. 21 shows that when an image receiverin a grid frame 702 is placed on a semi-erected metallic bed 705, areported radiation source position 701 can be slightly lower than anactual radiation source position 700. Actual and reported center x-raypositions 703 and 704 respectively differ.

The graph of FIG. 22 shows the amount of vertical displacement betweenreported and actual radiation source positions as a function of the gridframe angle relative to gravity. For this particular case therelationship can be approximated as a linear function of the grid frameangle, which can be measured by an inclinometer or accelerometer etcthat is attached to the grid frame. The compensation angle is added ontop of the reported radiation source position so as to achieve thecompensated values.

Another method embodiment to mitigate interference from the patient'sbed and other surrounding structures is selective use of sensors thatare least likely to be affected by the metallic structures.Experimentally, it has been found that proximity to the metal structureincreases interference effects; the closer the sensor is to the metalstructure, the more likely the sensor is to encounter higherinterference. Referring to FIG. 21, for example, sensors closest to thebed frame are subject to higher interference. The alignment accuracy canbe improved by ignoring sensors that are more susceptible tointerference due to their relative position with respect to the bedframe, when there are still enough of sensors remaining for calculationor, alternately, to weight sensor measurement data accordingly.

It should be observed that a number of component arrangements arepossible for emitter and sensing apparatus. For example, whileorthogonal sensor element placement has advantages and is generallystraightforward, other angular relationships could be used betweensensor elements that receive the same signal, with corresponding changesin how position is computed. In practice, angles should differ from onesensor element to the next by at least ten degrees. More than two sensorelements are generally grouped together for detecting the same generatedmagnetic field.

Operational modifications can also be made in exemplary embodimentsherein. For example, while sine and cosine signal processing is familiarand straightforward, other types of periodic signals could be used, suchas repeated triangular or square waves, for example. Even non-periodicwaveforms could be used if there is a way for the signal processor toidentify and correlate them with known signatures. The use of a carriersignal has advantages for allowing the sensor elements in sensingapparatus R to be tuned to the emitter carrier frequency. However, theuse of a carrier signal frequency is optional.

For triangulation between the x-ray source and receiver, at least oneemitter apparatus T is needed, with at least three sensors as part ofsensing apparatus R. Exclusive pairing of sensor elements to a specificemitter is not required. All sensor elements can be used to providesignals from each emitter T. Advantageously, no pointing vector isneeded; sufficient information for positioning within the standard sixDOF is provided by a combination of at least six measurements, obtainedfrom either one emitter (T) or two (T1, T2), wherein phase and amplitudeat one sensor element can be considered as separate measurements forthis purpose.

An embodiment of the present invention provides an apparatus foraligning a radiation source with an image receiver having at least afirst transmitter apparatus that is energizable to generate, aboutitself, a magnetic field having a fixed-position field pattern andhaving a time-varying vector direction at a predetermined frequency anda sensing apparatus with a plurality of sensor elements, wherein each ofthe plurality of sensor elements provides a sensor signal at thepredetermined frequency of the time-varying magnetic field and whereinat least two of the sensor signals from the plurality of sensor elementsdiffer from each other in phase or magnitude or both. There is a signalgeneration circuit that generates an output signal according to theplurality of sensor signals, wherein the output signal is indicative ofthe position and orientation of the sensing apparatus relative to the atleast the first transmitter apparatus. The at least first transmitterapparatus is coupled to either one of the radiation source and the imagereceiver and the sensing apparatus is coupled to the other one of theradiation source and the image receiver.

While the invention has been illustrated with respect to one or moreimplementations, alterations and/or modifications can be made to theillustrated examples without departing from the spirit and scope of theappended claims. In addition, while a particular feature of theinvention can have been disclosed with respect to only one of severalimplementations/embodiments, such feature can be combined with one ormore other features of the other implementations/embodiments as can bedesired and advantageous for any given or particular function. The term“at least one of” is used to mean one or more of the listed items can beselected. The term “about” indicates that the value listed can besomewhat altered, as long as the alteration does not result innonconformance of the process or structure to the illustratedembodiment. Finally, “exemplary” indicates the description is used as anexample, rather than implying that it is an ideal. Other embodiments ofthe invention will be apparent to those skilled in the art fromconsideration of the specification and practice of the inventiondisclosed herein. It is intended that the specification and examples beconsidered as exemplary only. The scope of the invention is indicated bythe appended claims, and all changes that come within the meaning andrange of equivalents thereof are intended to be embraced therein.

The invention claimed is:
 1. A method for aligning a radiation sourcewith a portable image receiver in a radiographic imaging system, themethod comprising: generating a magnetic field with a predeterminedfield pattern and with a time-varying vector direction at apredetermined frequency from an emitter apparatus that is coupled to theradiation source, the generated magnetic field comprising asynchronization signal; accessing sensed signals from the magnetic fieldfrom a sensing apparatus coupled to the image receiver, the sensingapparatus comprising three or more sensor elements, wherein at least twoof the sensor elements are arranged at different angles relative to eachother and are disposed outside the imaging area of the image receiver;and generating an output signal indicative of alignment according to theamplitude and phase of the obtained sensed signals relative to thesynchronization signal.
 2. The method of claim 1 wherein the outputsignal is further indicative of a distance between the emitter apparatusand the sensing apparatus according to the sensed output signals.
 3. Themethod of claim 1 wherein the magnetic field is a first magnetic field,the predetermined field pattern is a first predetermined field pattern,the time-varying vector direction at the predetermined frequency is afirst time-varying vector direction at a first predetermined frequency,the emitter apparatus is a first emitter apparatus, and thesynchronization signal is a first synchronization signal, the methodfurther comprising; generating a second magnetic field with a secondpredetermined field pattern and with a second time-varying vectordirection at a second predetermined frequency from a second emitterapparatus that is coupled to the radiation source, wherein the secondgenerated magnetic field further comprises a second synchronizationsignal.
 4. The method of claim 1 wherein generating the magnetic fieldfurther comprises generating a carrier signal.
 5. The method of claim 1wherein generating the magnetic field further comprises energizing amotor.
 6. The method of claim 1 wherein obtaining the sensed signalsfurther comprises iteratively comparing the obtained sensed signalsagainst a mathematical model of theoretical phase and amplitude forspatial disposition of the receiver relative to the source until abest-fit match is identified.
 7. The method of claim 1 furthercomprising processing the provided output signal to compensate forinterference from metal structures associated with a patient's bed. 8.The method of claim 1 wherein providing the output signal indicative ofthe alignment comprises weighting the obtained sensed signals accordingto locations of one or more of the sensor elements relative to apatient's bed.
 9. The method of claim 1 wherein providing the outputsignal comprises ignoring the obtained sensed signal from one or more ofthe sensor elements according to locations of the one or more of thesensor elements relative to a patient's bed.
 10. The method of claim 3wherein the first and second predetermined frequencies are the same. 11.The method of claim 3 further comprising displaying the alignment,wherein generating the first magnetic field comprises energizing a firstemitter coil and a second emitter coil at the same frequency, andwherein the first and second emitter coils are substantially orthogonalto each other.
 12. An apparatus for aligning a radiation source in aradiographic imaging system with a portable image receiver, theapparatus comprising: a first emitter apparatus coupled to the radiationsource, the first emitter apparatus comprises first and second coilsenergizable to generate a first magnetic field having a firstpredetermined field pattern and having a first time-varying vectordirection at a first predetermined frequency; a control circuit toprovide a first synchronization signal followed by the firstpredetermined field pattern; a sensing apparatus comprising a pluralityof sensor elements, spaced apart from each other, each of the pluralityof sensor elements provides a sensor signal at the first predeterminedfrequency and wherein at least two of the sensor signals from theplurality of sensor elements differ from each other in at least one ofphase and magnitude relative to the first synchronization signal; and asignal generation circuit that generates an output signal according tothe plurality of sensor signals, the output signal indicative of theposition and orientation of the sensing apparatus relative to theemitter apparatus.
 13. The apparatus of claim 12 further comprising: asecond emitter apparatus coupled to the radiation source, wherein thesecond emitter apparatus comprises third and fourth coils and isenergizable to generate a second magnetic field having a secondpredetermined field pattern and having a second time-varying vectordirection at a second predetermined frequency, and wherein the sensingapparatus further provides a sensor signal at the second predeterminedfrequency.
 14. The apparatus of claim 12 wherein the first and secondcoils are disposed substantially orthogonal to each other or the firstand second emitter coils differ from each other with respect to angularorientation by at least 10 degrees.
 15. The apparatus of claim 12further comprising an accelerometer coupled to either the first emitterapparatus or the sensing apparatus and wherein the accelerometer is incommunication with the signal generation circuit, wherein the pluralityof sensor elements comprise a coil, a Hall-effect device, a flux gate, amagneto-resistive sensor, a flux gate sensor, and a giantmagneto-resistive sensor.
 16. The apparatus of claim 12 wherein thefirst time-varying magnetic field is sinusoidal, and wherein theportable image receiver has an antiscatter grid.
 17. The apparatus ofclaim 12 wherein one or more sensor elements of the plurality of sensorelements are aligned in parallel with a metal feature on the receiver.18. The apparatus of claim 12 wherein at least one of the first orsecond emitter apparatus modulates a carrier signal that is at afrequency higher than the frequency of the first and second magneticfields with time-varying vector directions.
 19. An apparatus foraligning a radiation source in a radiographic imaging system with animage receiver, the apparatus comprising: a first emitter apparatus anda second emitter apparatus coupled to the radiation source, wherein atleast one of the first and second emitter apparatus is energizable togenerate: (i) a magnetic field having a fixed-position field pattern andhaving a time-varying vector direction at a predetermined frequency; and(ii) a synchronization signal; a control circuit that alternatelyenergizes the first emitter apparatus and the second emitter apparatusover different, substantially non-overlapping time intervals; a sensingapparatus comprising a plurality of sensor elements spaced apart fromeach other and disposed outside the imaging area of the image receiver,wherein each of the plurality of sensor elements provides a sensorsignal at the predetermined frequency of the time-varying magnetic fieldand wherein at least two of the sensor signals from the plurality ofsensor elements differ from each other in at least one of phase andmagnitude relative to the synchronization signal; and a signalgeneration circuit energizable to generate an output signal according tothe plurality of sensor signals, the output signal indicative of theposition and orientation of the sensing apparatus relative to at leastthe first emitter apparatus.
 20. The apparatus of claim 19 wherein thetime-varying magnetic field is sinusoidal or periodic, wherein at leastone of the first or second emitter apparatus modulates a carrier signalthat is at a frequency higher than the frequency of the magnetic fieldwith time-varying vector directions, wherein the imaging system furthercomprises an antiscatter grid.